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Major depressive disorder, also called "clinical depression" or often simply "depression", is a common, long-lasting and diverse psychiatric syndrome that significantly affects a person's thoughts, behavior, feelings and sense of well-being. Symptoms include low mood and aversion to activity. Depressed people may also feel sad, anxious, empty, hopeless, worried, helpless, worthless, guilty, irritable, hurt, or restless. They may lose interest in activities that once were pleasurable, experience loss of appetite or overeating, have problems concentrating, remembering details, or making decisions, and may contemplate or attempt suicide. Insomnia, excessive sleeping, fatigue, loss of energy, or aches, pains, or digestive problems that are resistant to treatment may also be present.[1]

About one in six people in the U.S will succumb to depression at some point during their life span,[2] and according to the World Health Organization, depression is projected to reach second place as leading contributor to the global burden of disease by the year 2020.[3] The current antidepressants have late drugefficacy about 3–6 weeks, besides, many patients’ illness cannot be alleviated.[4]

It is difficult to develop an animal model that perfectly reproduces the symptoms of depression in patients. Animals lack self-consciousness, self-reflection, and consideration; moreover, hallmarks of the disorder such as depressed mood, low self-esteem or suicidality are hardly accessible in non-humans. However, depression, as other mental disorders, consists of endophenotypes[5] that can be reproduced independently and evaluated in animals. An ideal animal model offers an opportunity to understand molecular, genetic, and epigenetic factors that may lead to depression. By using animal models, the underlying molecular alterations and the causal relationship between genetic or environmental alterations and depression can be examined, which would afford a better insight into pathology of depression. In addition, animal models of depression are indispensable for identifying novel therapies for depression.

An appropriate animal model of human depression should fulfill the following criteria as much as possible: strong phenomenological similarities and similar pathophysiology (face validity), comparable etiology (construct validity), and common treatment (predictive validity).[6][7][8] Again, depression is a heterogeneous disorder and its many symptoms are hard to be produced in laboratory animals. The question therefore remains whether we can know the animal is "depressed". Actually, few models of depression fully fit these validating criteria, and most models currently used rely on either actions of known antidepressants or responses to stress. It is not necessary for an "ideal" animal model of depression to exhibit all the abnormalities of depression-relevant behaviors, just like that the patients do not manifest every possible symptom of depression.

Antidepressant screening tests, not like the models which can be defined as an [organism] or a particular state of an organism that reproduces aspects of human pathology, provide only an end-point behavioral or physiological measure designed to assess the effect of the genetic, pharmacological, or environmental manipulation.

Forced-swimming test:[9] The forced-swimming test (FST) is based on the observation that animals develop an immobile posture in an inescapable cylinder filled with water. In this test, immobility is interpreted as a passive stress-coping strategy or depression-like behavior (behavioral despair). After antidepressant administration, the animals will actively perform escape-directed behaviors with longer duration than animals with control saline treatment. FST is the most widely used tool in depression research, more specifically as a screen for acute antidepressants.

The advantages of FST are that it is low-costing and is a fast and reliable tool, easy to handle and has proven its reliability across laboratories, for testing potential antidepressants activities with a strong predictive validity. Besides, it allows rapid screening of large numbers of drugs. The major disadvantages of FST are that it has poor face and construct validities. The test is sensitive to acute treatment only, and its validity for non-monoamine antidepressants is uncertain

Tail suspension test:[10] The TST, also known as tail suspension test, shares a common theoretical basis and behavioral measure with the FST. In the TST, mice are suspended by their tails using adhesive tape to a horizontal bar for a certain couple of minutes, and the time of immobility is recorded. Typically, the suspended rodents perform immediately escape-like behaviors, followed by developing an immobile posture. If antidepressants are given prior to the test, the subjects will be engaged in escape-directed behaviors for longer periods of time than after saline treatment, exhibiting a decrease in duration of immobility.

A major advantage of the TST is that it is simple and inexpensive. A major disadvantage of the TST is that it is restricted to mice and limited to strains that do not tend to climb their tail. Besides, like FST, TST is sensitive to acute treatment only, and its validity for non-monoamine antidepressants is uncertain.

Sucrose preference:[11]Rodents are born with an interest in sweet foods or solutions. Reduced preference for sweet solution in sucrose preference test represents anhedonia, while this reduction can be reversed by treatment with chronic antidepressants. This test can measure the affective state and motivation of subject rodents; however, further validation is needed for working as a model of depression.

Intracranial self-stimulation:[11]Intracranial self-stimulation (ICSS) can be utilized in rodents to understand how drugs affect the function of brain reward system. In this paradigm, in which the animal is trained to spin a wheel to receive a current through electrodes implanted in its own brain for rewarding hypothalamic stimulation. ICSS shares a common theoretical basis with the sucrose preference. Reduced preference for self-stimulating for reward cognition represents a loss of interest, fatigue and a loss of energy during depressive episodes, while this reduction can be reversed by treatment with antidepressants. Like sucrose preference test, ICSS can measure the affective state and motivation of subject rodents, and again, further validation is needed for working as a model of depression.

Novelty-induced hypophagia:[12]Hypophagia, one of the anxiety symptoms in rodents, is defined as the reduction in feeding in response to novelty, and it can be evoked by various novel features of the environment, including novel food, novel testing environment and novel food containers. Novelty-induced hypophagia (NIH) is a very recently developed test, which measures the latency and consumption of food in a novel unfamiliar environment. The test rather reflects the anxiolytic effects of antidepressants, and the response is seen only after chronic treatment with antidepressants rather than acute.

Open field:[13] Rodents tend to avoid brightly illuminated areas, and this avoidance is confused as a symptom of anxiety. Open field is a bright enclosure and during the test rodents are placed in this arena thus forced to interact with a novel and bright environment. The movement of the experimental subject will be recorded in distance and pathway.

Elevated plus maze:[13] For the elevated plus maze test, the rodents are placed at the intersection of the four arms of the maze (two open, two closed), facing an open arm. The number of entries and time spent in each arm is recorded and valid results are obtained in a single 5-minute testing session. An increase in the open-arm time is an index of anti-anxiety behavior of rodents.

Dark/light box:[13] The dark/light box test is also based on the rodents' innate aversion to brightly illuminated areas and on the spontaneous exploratory behaviour of the animals. A natural conflict situation occurs when an animal is exposed to an unfamiliar environment or novel objects. The conflict is between the tendency to explore and the initial tendency to avoid the unfamiliar. The exploratory activity reflects the combined result of these tendencies in novel situations. The test apparatus of dark/light box consists of a dark compartment and an illuminated compartment. Drug-induced increases in behaviors in the white part of a two-compartment box are suggested as an index of anxiolytic activity.

Open field test, elevator plus maze test, and dark/light box test can work as an antidepressant screen by measuring anxiety-related behavior as an accompanying endophenotype of depression. It is known that some antidepressants administration will cause a decrease behavior in these tests just like anxiolytics. However, the response to some antidepressants couldn’t be detected. Besides, these tests has their own problem. It is difficult to discriminate decreased anxiety-related avoidance from increased novelty-seeking in these tests.

Certain types of human depression are precipitated by stressful life events, and vulnerable individuals experiencing these stressors may develop clinical depression. Consequently, the majority of animal models of depression are based on the exposure to various types of acute or chronic stressors.

Learned helplessness:[14][15][16][17][18] The learned helplessness model (LH), one of the well validated animal models, is the best replicated one. The rationale is that exposure to uncontrollable and stressful life events makes people to feel like losing control, and sometimes leads to a depressive like behavior ultimately.The model is based on the observation that animals also develop deficits in escape, cognitive and rewarded behaviors when they have been subjected to repeated unavoidable and uncontrollable shocks. LH is induced in one day or over several days of repeated inescapable stress by the treating of tail shock or foot shock in shuttle boxes. Helpless behavior is evaluated by analyzing the performance in an active escape test, such as the latency to press a lever or cross a door.

An advantage of LH is that the cognitive and other behavioral outcomes seem to be correlated, thus helping to understand the depressive symptomatology in humans. Besides, this model can also be generally used to measure the escape performance of mice with different mutations, in which target genes of depression may affect the vulnerability to develop a depressive-like state. These excellent face and predictive validities make LH an interesting model to explore the pathophysiology of depression. The biggest disadvantage of LH is it requires very strong stressors to induce the behavioral phenotypes, which does raise ethical problems. Also, most of the symptoms do not persist long enough following cessation of the uncontrollable shock.

Chronic mild stress:[19][20][21][22] The chronic mild stress (CMS) model is probably the most valid animal model of depression. It aims to model a chronic depressive-like state that develops gradually over time in response to stress, and they can provide more natural induction. CMS involves the exposure of animals to a series of mild and unpredictable stressors (periods of food and water deprivation, small temperature reductions, changes of cage mates, and other similar individually innocuous manipulations) during at least 2 weeks. The model has been reported to result in long lasting changes of behavioral, neurochemical, neuroimmune, and neuroendocrinological variables resembling reward functions including decreased intracranial self-stimulation, reflecting anhedonia that is reversed by chronic but not acute antidepressant treatment.

The advantages of this model are its good predictive validity (behavioral changes are reversed by chronic treatment with a wide variety of antidepressants), face validity (almost all demonstrable symptoms of depression have been reproduced), and construct validity (CMS causes a generalized decrease in responsiveness to rewards). However, there is a common practical difficulty in carrying out CMS experiments, which are labor intensive, demanding of space, and of long duration. Besides, the procedure can be difficult to be established and data can be hardly replicated.

Social defeat stress:[23][24][25][26]Social defeat stress (SDS) is a chronic and recurring factor in the lives of virtually all higher animal species. Humans experiencing social defeat show increased symptoms of depression, loneliness, anxiety, social withdrawal and a loss of self-esteem. Since the majority of stress stimuli in humans that lead to psychopathological changes are of social nature, SDS model have gained increasing attention as they might render useful to study certain endophenotypes of depression. During the stress period, the male rodent will be introduce into a different territory of other cospecific males for each day as an intruder, that cause it to be investigated, attacked and defeated by the residents. The consequent behavior changes in the subject caused by SDS, like decreased social interaction or lacking of interesting, are similar to some parts of human depression, and behavioral treatment and antidepressants can reverse these changes in SDS model.

Like CMS, SDS has good predictive validity (behavioral changes are reversed by chronic treatment with a wide variety of antidepressants), face validity (many symptoms of depression have been reproduced), and construct validity(causing a generalized decrease in responsiveness to rewards) and gives another validity that only chronic but not acute antidepressant administration can reverse the social aversion. One disadvantage of SDS model is the long duration. To apply SDS model for studying human depression, the period of it should last at least 20 days otherwise only anxiety symptoms could be induced. Worthy of note, only male rodents can be used for this model, since female rodents do not fight each other in a resident–intruder confrontation.

Early adverse experiences such as traumatic life events in childhood result in an increased sensitivity to the effects of stress later in life and influence the individual vulnerability to depression.[27] Suitable animal models could provide a basis for understanding potential mechanisms of environmental and developmental factors of individual differences in stress reactivity and vulnerability to disorders. Models of early life stress involve prenatal stress, early postnatal handling and maternal separation. All these treatments have been demonstrated to produce significant effects that last until adulthood.

Maternal deprivation:[28] The maternal deprivation model is the most widely used early life stress model. This model manipulates the maternal separation of early life deprivation, in which pups are separated from the dam for 1–24 h per day during the first two postnatal weeks. Maternal separation results in increased anxiety- and depression-like behaviors and increased HPA response in adulthood.

Olfactory bulbectomy:[29] The olfactory bulbectomy in rodents results in a disruption of the limbic-hypothalamic axis with the consequence of behavioral, neurochemical, neuroendocrine and neuroimmune changes, of which many resemble changes seen in depressed patients. It is still not clear how bulbectomy in animals actually relates to depression in humans. It might simply result from a high intensity of chronic stressor caused by chronic sensory deprivation. This model shows high predictive validity as it mimics the slow onset of antidepressant action reported in clinical studies, responding chronic but not subchronic antidepressant treatment with no response to other drugs. It is worth to be mentioned that, unlike stress-related models, the rat in lesion model represents an agitated, hyposerotonergic depression-related phenotype, rather than a retarded depression.

Psychostimulant withdrawal (amphetamine, cocaine):[30] In humans, withdrawal from chronic psychostimulants generates symptoms that have strong behavioral and physiological parallels to depression. Therefore, the examination of the behavioral effects of drug withdrawal in rodents may provide insights into the underlying neurobiological mechanisms and aid in the development of animal models of depression that are sensitive to antidepressant agents. Following withdrawal from drugs such as amphetamine or cocaine, rodents display behavioral changes that are highly similar to some aspects of depression in humans, such as anhedonia, and behaviors opposite to those seen after treatment with antidepressant drugs.

Forward genetics:[34] Forward genetics allows identifying relevant genes without any prior knowledge of genetic to the phenotype. Large scale random mutagenesis screens, like ENU, have resulted in a great number of mutants displaying depression or antidepressant-like behavior.